我对PostgreSQL 中 index scan 与 seq scan 的对比学习
开始数据量很小的时候，我们可以看到，seq scan 比 index scan 更加有效。那是因为 index scan 至少要发生两次I/O，一次是 读取索引块， 一次是读取数据块。当index 很大的时候，情况可能会更加复杂。
postgres=# select a.relpages, a.reltuples, a.relfilenode,a.reltype,b.typname from pg_class a, pg_type b where a.relname like 'gaotab%' and a.reltype=b.oid;
relpages | reltuples | relfilenode | reltype | typname
----------+-----------+-------------+---------+---------
1 | 100 | 16396 | 16386 | gaotab
数据量为 100条记录。
预估成本：
postgres=# set session enable_seqscan=false;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN
---------------------------------------------------------------------
Index Scan using idx_id on gaotab(cost=0.00..8.27 rows=1 width=5)
Index Cond: (id = 50)
(2 rows)
postgres=# set session enable_seqscan=true;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN
------------------------------------------------------
Seq Scan on gaotab(cost=0.00..2.25 rows=1 width=5)
Filter: (id = 50)
(2 rows)
实际执行：
postgres=# set session enable_seqscan=false;
SET
postgres=# explain analyze select name from gaotab where id=50;
QUERY PLAN
--------------------------------------------------------------------------------
-------------------------------
Index Scan using idx_id on gaotab(cost=0.00..8.27 rows=1 width=5) (actual tim
e=0.112..0.113 rows=1 loops=1)
Index Cond: (id = 50)
Total runtime: 0.133 ms
(3 rows)
postgres=# set session enable_seqscan=true;
SET
postgres=# explain analyze select name from gaotab where id=50;
QUERY PLAN
--------------------------------------------------------------------------------
----------------
Seq Scan on gaotab(cost=0.00..2.25 rows=1 width=5) (actual time=0.014..0.018
rows=1 loops=1)
Filter: (id = 50)
Rows Removed by Filter: 99
Total runtime: 0.034 ms
(4 rows)
等到数据量大的时候，就是截然不同了。
数据为1000条记录时，通过查询可以看到，已经跨越了7个page：
postgres=# analyze;
ANALYZE
postgres=# select a.relpages, a.reltuples, a.relfilenode,a.reltype,b.typname from pg_class a, pg_type b where a.relname like 'gaotab%' and a.reltype=b.oid;
relpages | reltuples | relfilenode | reltype | typname
----------+-----------+-------------+---------+---------
7 | 1000 | 16396 | 16386 | gaotab
(1 row)
postgres=#
再次预估成本，此时seq scan 已经开始变得不划算了：
postgres=# set session enable_seqscan=false;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN
---------------------------------------------------------------------
Index Scan using idx_id on gaotab(cost=0.00..8.27 rows=1 width=6)
Index Cond: (id = 50)
(2 rows)
postgres=# set session enable_seqscan=true;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN
---------------------------------------------------------------------
Index Scan using idx_id on gaotab(cost=0.00..8.27 rows=1 width=6)
Index Cond: (id = 50)
(2 rows)
postgres=# set session enable_indexscan=false;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN
---------------------------------------------------------------------
Bitmap Heap Scan on gaotab(cost=4.26..8.27 rows=1 width=6)
Recheck Cond: (id = 50)
->Bitmap Index Scan on idx_id(cost=0.00..4.26 rows=1 width=0)
Index Cond: (id = 50)
(4 rows)
postgres=# set session enable_bitmapscan=false;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN
-------------------------------------------------------
Seq Scan on gaotab(cost=0.00..19.50 rows=1 width=6)
Filter: (id = 50)
(2 rows)
postgres=#
实际执行
postgres=# set session enable_seqscan=false;
SET
postgres=# explain analyze select name from gaotab where id=50;
QUERY PLAN
------------------------------------------------------------------------------------------------------------
-----------------------
Index Scan using idx_id on gaotab(cost=10000000000.00..10000000008.27 rows=1 width=6) (actual time=0.020.
.0.022 rows=1 loops=1)
Index Cond: (id = 50)
Total runtime: 0.051 ms
(3 rows)
postgres=# set session enable_seqscan=true;
SET
postgres=# set session enable_indexscan=false;
SET
postgres=# set session enable_bitmapscan=false;
SET
postgres=# explain analyze select name from gaotab where id=50;
QUERY PLAN
-------------------------------------------------------------------------------------------------
Seq Scan on gaotab(cost=0.00..19.50 rows=1 width=6) (actual time=0.015..0.095 rows=1 loops=1)
Filter: (id = 50)
Rows Removed by Filter: 999
Total runtime: 0.109 ms
(4 rows)
postgres=#
[作者：技术者高健@博客园mail: luckyjackgao@gmail.com ]
结束
页:
[1]